[PATCH 4/4] mm: remove kernel address exposure in free_reserved_area()

Josh Poimboeuf
2016-10-25T14:51:31Z

Linus suggested we try to remove some of the low-hanging fruit related
to kernel address exposure in dmesg. The only leaks I see on my local
system are:

Freeing SMP alternatives memory: 32K (ffffffff9e309000 - ffffffff9e311000)
Freeing initrd memory: 10588K (ffffa0b736b42000 - ffffa0b737599000)
Freeing unused kernel memory: 3592K (ffffffff9df87000 - ffffffff9e309000)
Freeing unused kernel memory: 1352K (ffffa0b7288ae000 - ffffa0b728a00000)
Freeing unused kernel memory: 632K (ffffa0b728d62000 - ffffa0b728e00000)

Linus says:

"I suspect we should just remove [the addresses in the 'Freeing'
messages]. I'm sure they are useful in theory, but I suspect they
were more useful back when the whole "free init memory" was
originally done.

These days, if we have a use-after-free, I suspect the init-mem
situation is the easiest situation by far. Compared to all the dynamic
allocations which are much more likely to show it anyway. So having
debug output for that case is likely not all that productive."

With this patch the freeing messages now look like this:

Freeing SMP alternatives memory: 32K
Freeing initrd memory: 10588K
Freeing unused kernel memory: 3592K
Freeing unused kernel memory: 1352K
Freeing unused kernel memory: 632K

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-mm@kvack.org
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
mm/page_alloc.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2b3bf67..3f63973 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6508,8 +6508,8 @@ unsigned long free_reserved_area(void *start, void *end, int poison, char *s)
}

if (pages && s)
- pr_info("Freeing %s memory: %ldK (%p - %p)\n",
- s, pages << (PAGE_SHIFT - 10), start, end);
+ pr_info("Freeing %s memory: %ldK\n",
+ s, pages << (PAGE_SHIFT - 10));

return pages;
}
--
2.7.4